Approximate glyph size calculations for Label's and their
bounding boxes.

Calculations are based on metrics derived from the Courier
font. As Courier is a monospaced font, applying these metrics
to other font families will usually produce over-estimates
(bounding boxes will be longer than the true visual length
of the text). Furthermore, even italic or bold Courier will
have different metrics.

This is a deficiency of Wumpus, and limits its text handling
capabilities - for example, text cannot be reliably centered
or right aligned as its true length is not known. However, more
powerful alternatives would need access to the metrics embedded
within font files. This would require a font loader and add
significant implementation complexity.